When my friend plugs his flash drive in to his computer, The computer wont read it but when he puts in a different computer, the PC reads it. What do you think is the problem?

Did other switches work on this USB port? I had the same problem and at my computer the USB port was crashed.

Maybe your friend you try the flash drive on another port of the computer to make sure, if this a hardware problem or a damage in the software.

If the computer cant read your flash drive, it may be that your flash drive is one of the newer type and your computer is not compatible with it. It may also be that some of the settings in your computer needs to be changed.

The problem might have been with the USB bus peripheral in the system. If the system is old and haven't yet been rebooted atleast once then it is time to reinstall those drivers and OS. It would most probably solve your problems.